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Magnetic diffusion time in nonlinear steel is shown to vary with applied field magnitude and whether the field is undergoing infusion (inward flow upon turnon) or effusion (outward flow upon turnoff). Transient finite-element analysis is used to compute planar flux effusion that agrees very closely with an analytical equation for unsaturated steel slabs. When the steel is saturated, it is shown that effusion times can be much longer than infusion times, which are greatly reduced by saturation. Effusion times are computed for varying levels of saturation both in planar slabs and in axisymmetric cylinders.